Multicasting is a method, by which one and the same content can be distributed efficiently to several recipients in a network. In comparison with conventional transmission between two parties, referred to by the abbreviation PTP (point to point), multicasting saves bandwidth, because in IP networks, for instance, packets with an identical content are not transmitted from one source to several recipients over the entire route, but one packet is transmitted from the source and multiplied in the last possible router in the network into as many copies as there are recipients. Thus, this is a PTM (point to multipoint) transmission. This transmission can be simultaneous, contrary to PTP transmission, in which the transmission is to one recipient at a time.
A certain section of the IP address space is reserved for multicast addresses and these addresses are not given as normal IP addresses of devices. A given multicast address belongs to all devices belonging to the group in question. The basic idea in multicasting is that the recipients register to desired groups by using a known protocol (Internet Group Management Protocol, IGMP). Separate multicasting protocols deliver a multicast transmission (UDP-based traffic from one to many) from the sender to the subscribers.
Multicasting is implemented by means of multicast trees. Each multicast sender has a multicast tree. A multicast tree refers to a connection from a sender through different routers and branching off from the routers to each recipient.
The known method for implementing multicasting causes problems in new data networks. This concerns especially networks that comprise unidirectional links. One such network is the multi-bearer network (MBN) that is a network arrangement providing several different network service types for the delivery of data packets. The network comprises a core network that is connected to the Internet or some other public data network and through interface units to different network services, i.e. access networks, such as the cellular radio networks GSM (Global System for Mobile Communication), GPRS (General Packet Radio System) and UMTS (Universal Mobile Telecommunications System), and the broadcast networks DAB (Digital Audio Broadcast) and DVB (Digital Video Broadcast). Of the above networks, the cellular radio networks are bi-directional and the broadcast networks unidirectional.
A subscriber terminal connected to a system through a unidirectional link cannot connect to a multicast transmission organized by using the prior art, because it is unable to transmit anything to the network due to the unidirectional link that has no transmit capability.